Stupidity: When Bruce, Betty and She-Hulk are infiltrating Sea-Base, they do so by docking their submarine on top and sneaking in. For what is supposed to be a top secret S.H.I.E.L.D. base, they surprisingly have no security measures - such as cameras and sensors - to detect when an outside force is in the vicinity.
Stupidity: When Dark Hulk throws the tank at the approaching jeeps, the drivers for all three jeeps slam on their brakes in order to bail out of the vehicles in the nick of time when they could have just turned the jeeps to avoid being hit.
Darkness and Light: Part 1 - S1-E11
Stupidity: When the Leader's spy is reporting to him, he removes his human mask for the benefit of the audience. This is a pretty foolish thing to do considering he's in a top secret military base, where he can be seen by anyone that happens to be in the area, or potentially by security cameras.
Darkness and Light: Part 1 - S1-E11
Stupidity: When Doc Samson emerges from the cave system and into the laboratory grounds, he says to the others "All clear" without even looking behind him to see if anyone is there, which leads to them being seen by Gabe Jones.
Answer: The Hulk is a giant brute who smashes everything in his path when he is angry. In Ang Lee's "Hulk," General Ross, played by Sam Elliot, explains it to his daughter.